Many satellite communications systems require multiple beams to be placed over a geographic area. FIG. 3, for example, illustrates a pattern of coverage to provide service to the United States from a geostationary satellite located at 91 degrees west longitude. Numerous narrow beams may be formed from a relatively few elementary feeds by a process known as beamforming and described, for example, in U.S. Pat. Nos. 5,115,248 and 5,784,030. FIG. 3, for example, shows a pattern of 135 spot beams created from a feed array having only 48 elements. Adaptive beamforming permits electrical reconfiguration of the direction of each spot beam, or the formation of beams with different sizes and shapes, each accomplished without the need to change any hardware element.
A beamforming capability provides important benefits to many satellite payloads. For example, it permits a given satellite to operate from a number of different orbital locations. Thus, a satellite fleet operator licensed to operate geostationary spacecraft at multiple orbital locations may use a common hardware design for all locations and electrically configure the beam as required to tailor the spot beam pattern based on the satellite's location. Moreover, beamforming allows a satellite, which typically has a fifteen year life span, to be adapted on orbit to changing traffic patterns or new applications on the ground.
Beamforming, however, is technically challenging to perform on a satellite, inasmuch as the amplitude and phase relationship of each feed element within an array must be precisely set and provide for both the forward (gateway to satellite to user) signal path and the return (user to satellite to gateway) signal path. Conventional spacebased beamforming techniques include analog and digital beamforming networks (BFN's). Analog BFN's are generally co-located with the feed array, because it is otherwise difficult to compensate for losses or electrical path length variations between the feed apertures and the points of application of the beamforming coefficients. Volume and thermal constraints limit the number of analog BFN's that can be co-located with the feed array.
Digital BFN's have a better ability to compensate for losses or electrical path length variations between the feed apertures and the points of application of the beamforming coefficients. Accordingly, they can be employed in the middle of the payload at a considerable electrical path distance from the feed array, provided that strict attention is paid to design practices minimizing amplitude and phase variations and calibration processes that accurately track the variations.
The burdens associated with space-borne BFN's can be substantial, and include system reliability degradation, and added hardware mass, cost, power consumption and thermal control requirements. Moreover, if the BFN is on the satellite, the ability to introduce improved technologies and react flexibly to changing market demand is limited during the life of the satellite. Moving BFN functions to the ground is therefore desirable, but ground-based beamforming systems must overcome several additional problems not inherent in space-based beamforming. Among these are the need to compensate for gateway and satellite component performance changes over temperature and life, satellite and ground station pointing errors, and signal propagation amplitude and phase dispersion effects, including Doppler shifts.
These difficulties have limited the use of ground-based beamforming techniques. Known prior art techniques apply beamforming in only the return direction, or are limited to systems in which the feeder link signals are code division or time division multiplexed. Frequency division multiplexing is more commonly used in space, and offers significant cost and reliability advantages over code division and time division multiplexing.
The present invention provides for ground-based beamforming for both the forward and return communications path. The invention further provides for ground-based beamforming that can be employed in a system employing frequency division multiplexed signals.